Turbulence dynamics and ventilation pathways in a vegetated real-world urban canyon: A hydraulic flume study

This study investigates the aerodynamic impact of roadside trees on ventilation and turbulence dynamics within a real-world urban canyon in Bologna, Italy, utilizing high-resolution hydraulic flume experiments. Fluid velocity fields were captured via Feature Tracking technique across multiple vertical and horizontal planes, yielding a comprehensive three-dimensional representation of the flow topology. The experimental results reveal that the tree-free configuration exhibits ventilation pathways typical of asymmetrical urban geometries, characterised by highly three-dimensional features, localized high-velocity corridors, and large-scale vortical structures. Conversely, in the vegetated configuration, the aerodynamic drag and momentum dissipation induced by the tree crowns profoundly alter the flow field, suppressing the formation of major in-canyon vortices. While this structural barrier leads to a systemic reduction in both mean velocity (a decrease of ∼40%) and turbulence kinetic energy within the canyon, it simultaneously induces a distinct flow-channelling effect beneath the canopy (the magnitude of the velocity increases by approximately 200% in the area corresponding to the trunks). Furthermore, a quadrant analysis was deployed to identify and quantify the discrete sweep and ejection events driving turbulent momentum exchange across the canyon-atmosphere interface. Ultimately, this study provides a high-fidelity, high-resolution experimental dataset that serves as a rigorous benchmark for validating microscale numerical and Computational Fluid Dynamics (CFD) models designed to assess vegetation-atmosphere interactions in complex urban environments.